Isobutylene is a useful chemical raw material that is used in the manufacture of products such as methyl tertiary butyl ether and/or ethyl tertiary butyl ether, which are used as gasoline components; polyisobutylene that is used as a rubber and as an adhesion promoter as well as in the production of dispersants used in lubricating oils; and di-isobutylene that is used as an intermediate for surfactants and carboxylic acids. Isobutylene is also used as a raw material in the production of tertiary butyl alcohol.
Isobutylene is generally present as a component in the C4 cuts that are obtained from steam cracking and catalytic cracking refinery processes. These cuts typically contain a mixture of C4 saturated materials, particularly isobutane and normal butane, and C4 unsaturated materials including butadiene, normal butenes including both butene-1 and butene-2 and isobutylene. The butadiene may be removed by extraction or reaction, or converted by selective hydrogenation to produce a stream which contains predominantly normal butenes, isobutylene (or isobutene), and butanes; such a stream is sometimes known as raffinate-1. The isobutylene content of a stream may be determined by using conventional gas chromatographic techniques.
Butene streams are used as raw materials for hydroformylation to produce valeraldehyde. The valeraldehyde may then be dimerised and the product of dimerisation hydrogenated to produce 2 propyl heptanol or mixtures thereof with other alcohols which are finding use as alcohols in esterification reactions to produce plasticiser esters. Alternatively valeraldehyde may be hydrogenated to produce pentanol or amyl alcohol or mixtures of different isomers thereof which may be used as a solvent or in the production of zinc dialkyl dithiophosphates. The valeraldehyde may also be oxidised to produce valeric acid or isomer mixtures thereof which may be used in synthetic ester lubricant production. However, in order to obtain these products it is important that the alcohol produced in the hydroformylation reaction is 1 and/or 2 valeraldehyde,
which are obtained by the hydroformylation of butene-1 or butene-2. It has been preferred that the product is substantially free of 3-methylbutanal, which is the primary product obtained by the hydroformylation of isobutylene, and which is extremely difficult to separate from the other valeraldehydes, especially from 2-valeraldehyde. As is described in U.S. Pat. No. 4,287,370 the presence of isobutylene in hydroformylation reactions can also lead to the formation of undesirable resins.
Hydroformylation reactions of lower olefins such as ethylene, propylene and butenes have generally employed rhodium catalyst stabilised by phosphorus containing ligands operated in what is known as the low pressure oxo technology originally developed by Union Carbide Corporation and currently available under licence from Davy Process Technology Ltd. In another hydroformylation technology, cobalt containing catalysts are used and the process is operated at higher pressures. Exceptionally, rhodium catalyst is also used in high pressure processes, typically without the phosphorus ligand or with only a weak kind of ligand. When using high pressure technologies to hydroformylate mixed butene streams all the unsaturated materials are reacted leading to the presence of undesirable aldehydes and resins from the isobutylene. Generally when using rhodium based low pressure technology to hydroformylate butenes, it has been common practice to separate the isobutylene from the normal butenes in the raffinate-1 to produce a stream containing only n-butenes (butene-1 and butene-2, cis and trans) which can be used for hydroformylation. Such a stream is sometimes known as raffinate-2. The separation of isobutylene from raffinate-1 is however not an easy process and is expensive and time consuming. If fractionation is used a mixture of butene-1 and isobutene tends to be obtained overhead because butene-1 and isobutylene are difficult to separate, whilst the butene-2 tends to be obtained at the bottom of the fractionation tower because it is more easily separated from isobutylene and butene-1. Clearly this is not beneficial if one wants to obtain a butene-1 stream that is substantially free of isobutylene. Accordingly superfractionation may be used; however, this has very high energy requirements and is expensive to operate and complex to design. Furthermore, even the use of superfractionation may not result in complete separation of the isobutylene.
U.S. Pat. No. 6,100,432 shows the separation of isobutylene from raffinate-1 to produce raffinate-2 prior to hydroformylation with a rhodium catalyst. U.S. Pat. No. 4,287,370 states that the C4 feed to hydroformylation should contain no more than 1 wt % isobutylene. Similarly United States Patent Publication 2003/0022947 A1 discloses hydroformylation of raffinate-2, an isobutene depleted stream said to contain no more than 5 mol % isobutene. In this patent only the butene-1 is hydroformylated, the butene-2 and the isobutylene being substantially unreacted. An article by Walter J Scheidmeir of BASF in Chemiker-Zeitung 96 Jahrgang (1972) Nr 7. pp 383-387 shows the hydroformylation of a butene stream containing isobutylene in which all the unsaturated materials including the isobutylene, are converted. U.S. Pat. No. 6,555,716 describes a process in which olefins including raffinate-1 are fed to a tubular hydroformylation reactor which employs a rhodium catalyst in combination with a water soluble ligand, i.e. trisulphonated triphenylphosphine. In this process, high catalyst recycle volumes are required and a higher pressure is employed. In the process of U.S. Pat. No. 6,555,716 butene-1 is selectively hydroformylated whereas butene-2 remains unconverted and isobutylene is partially converted. The examples of U.S. Pat. No. 6,555,716 show that the ratio of the conversion of butene-1 and isobutylene is such that the hydroformylation does not effectively separate butene-1 and isobutylene.